The behavior of velocity of ultrasound in glasses below 3 K is analysed by combining the tunneling model and the infrared divergence theory. The variation of sound velocity is regarded as consisting of two parts, i.e., non-relaxation process and relaxation process. The former is analysed by the Bose elementary excitation theory and the latter by the theory of tun neling relaxation with infrared divergence. The theoretical curve of sound velocity vs temperature is consistent with the experimental data. The observed In T rule of sound velocity for normal frequency (107Hz) at 0.3-1K, and the phenomenon of appearance of sound velocity minimum for ultrahigh frequency (2GHz) belowe 0.1 K are also explained.